Answers for "How to get an Object to move physically correct (velocity)"
http://answers.unity.com/questions/1516719/how-to-get-an-object-to-move-physically-corect-vel.html
The latest answers for the question "How to get an Object to move physically correct (velocity)"Answer by Bunny83
http://answers.unity.com/answers/1516806/view.html
Unity's physics system is not very good at simulating rollercoaster mechanics. The physics system is great for low moderate velocities and forces and for handling collisions between rigidbody objects and calculating collision responses. However almost none of that is necessary for a rollercoaster. The extremely high forces and velocities you have to deal with would become totally unstable.
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To simulate a rollercoaster i would use a linearized spline path. [This gamedev question][1] has some great links on how to do this. Once you have a perfect linear mapping between the worldspace distance and the logical distance on our path the whole problem becomes one dimensionally as you can precalculate many things of the 3d environment like the affect of gravity based on the position along the spline path. You simply need to iterate the whole path at tiny steps and calculate the slope / angle to know how much influence gravity has at a point. You can store those samples in an animation curve which you can easily sample. If you store the effect of gravity as the local acceleration at this point you naturally get acceleration when going downhill and deceleration (negative acceleration) when going uphill.
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So each wheel group would simply be a point on the spline. It's position would just be a single float value. Likewise the velocity also would just be a single float as we reduced the problem to a one dimensional problem.
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You most likely want to use two parallel spline paths or one path and a normal vector at each point to specify the rotation around the spline path (in order to get a stable rotation and to be able to do screw motions, upside-down, etc). The normal approach as several advantages. Seperate splines have the problem that you need to take care of syncing the left and right track so the right side doesn't get "faster" than the left. Also the relative distance between the two tracks would be difficult to get right if you place the splines manually.
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Things like expected g-forces / centrifugal forces can also be precalculated and sampled based on the position on the path and the current velocity.
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This way you don't have to care about the wagon wheel to track collision and all the high forces that in real-life would be responsible for keeping the wagons on the track. This is basically a seperation of the pure rollercoaster mechanic which is one dimensional and the actual visual representation of the simulation.
[1]: https://www.gamedev.net/forums/topic/470537-constant-speed-moving-on-spline/Mon, 11 Jun 2018 06:43:50 GMTBunny83